The extensive development and improvement of xerographic methods of photocopying documents has led, in recent years, to renewed attempts to exploit the special properties of this process for both industrial and medical radiography. Early attempts to introduce the method were unsuccessful, principally because the sensitivity of the selenium plates was then too low to compete with film but also because the associated apparatus was not sufficiently convenient or reliable in every day use. Additionally, the commercial return was judged unattractive by the manufacturers.
In xeroradiography the detecting medium is a thin film of a photoconductor; a substance, that is, which normally contains very few charge carriers but in which radiation can liberate new carriers with a lifetime long enough to enhance greatly the natural conductivity. The surface of the photoconducting layer is first given a uniform surface charge, usually laid down by a corona charging device, and has been enclosed in a light-tight cassette and exposed to the radiation penetrating through the irradiated object. The original uniform charge is partly dissipated by the current thus induced in the photoconducting layer and the residual charge pattern on the object forms a "latent image" of the radiation. This charge pattern must then be made visible by some appropriate method of development. There are many ways of doing this, but the method usually employed in radiographic work is to expose the electrostatic "latent image" to an aerosol of electrically charged powder particles. These adhere to the surface principally in the regions of high field strength and thus delineate clearly any sharp steps or steep gradients in the charged density.
The conductivity induced in selenium by visible light has been studied very extensively. Very little conductivity is produced in vitreous selenium by light of wavelengths longer than 5,500 angstroms. The absorption coefficient and the induced conductivity both rise rapidly as the wavelength is reduced and below 4,000 angstroms the quantum yield (carrier pairs per photon absorbed) reaches unity at high field strengths. It cannot rise above unity since a single quantum of light in the visible or near ultraviolet can produce, at most, one pair of carriers. Blue light is strongly absorbed and the photons cannot penetrate beyond a very thin "photon absorption layer", within which all the charge carriers, positive holes and negative electronics, are formed. It was only until a few years ago that the observed strong dependence of conductivity upon field strength in the selenium could be explained very satisfactorily by two processes: recombination of positive and negative charge carriers within the photon absorption layer (where both kinds are present) and trapping of those carriers which escaped from the photon absorption layer as they were driven through the bulk of the selenium under the influence of the field.
U.S. Pat. No. 4,521,808, issued on June 4, 1985 to the University of Texas System describes an electrostatic imaging apparatus having a plate structure sequentially arranged as follows: electrode, insulator, photoconductive layer, diode layer, and electrode. In this invention, the diode layer is a layer of aluminum oxide that acts as a blocking contact between the electrode layer and the photoconductive layer. This very thin aluminum oxide layer acts as a blocking contact to retain the positive charge on the surface of the selenium. In this configuration, the blocking layer had a blocking potential that would have to be overcome for the current to flow through the contact. Unfortunately, after much experimentation with such an arrangement, it was found that a thin, aluminum oxide "diode layer" would create carriers between the photoconductive layer and the metallic electrode layer. The creation of these carriers reduces the hold time of the charge on the surface of the photoconductive layer, causes leakage across the diode junction, and requires a complementary output readout. It was found that such a configuration would not hold the charge sufficiently long to enable a readout over a large plate structure surface area. Additionally, the need to read the complementary output would distort the image by receiving large signals that would otherwise be categorized as "noise". This creates some distortion and abnormalities in the final image.
The subject of U.S. Pat. No. 4,521,808 was a electrostatic imaging plate known as the "Anderson System". A problem with the Anderson System, following experimentation, indicated that it was difficult to use as a large-area detector because of the amount of capacitance that builds up in the outer conductive layers of the plate structure. In essence, the capacitance in the large-area surface creates a large amount of noise relative to the signal produced by the readout techniques described in this patent.
The IBM technical Disclosure Bulletin, Volume 9, No. 5, of October 1966, pages 555 and 556, discloses a charge-storage beam-addressable memory. This device is a sandwich structure of semi-conductive and insulating materials. This structure is irradiated at selected points or under the influence of applied electric fields to store charges representing data bits. The reading out of the stored bits is accomplished by irradiating the semiconductor at selected points and observing the resultant discharge current. This memory structure has a configuration sequentially organized as follows: electrode, insulator, semiconductor, insulator, and electrode. The semiconductor material used in amorphous selenium. To perform writing, the beam is directed at any selected spot in the semiconductor while the device applies an electric field of selected polarity, representing one or zero, across the electrode layers. The semiconductor becomes locally conductive at the point where it is addressed by the beam, causing localized charges to be built up in the insulating layers at that point under the influence of the applied field. These local charges are trapped, at least for a limited time, when the beam is removed and the semiconductor reverts to its normal state.
The plate structure described in the IBM Technical Disclosure Bulletin is a digital memory storage system that looks for polarity output. This would not work effectively where amplitude output and imaging needs were required. In essence, the IBM system is looking for the existence of a signal or looking for the polarity output. As such, the system does not require the use of true insulators. The insulator used in this IBM structure is a made of a "defect" material that causes the charge to be trapped within the insulator rather than at the interface of the insulator and the photoconductive layer. In addition, it is difficult to construct the apparatus of the IBM disclosure since the tantalum or niobium oxides used in the insulating layer would interrupt the structure of the selenium. This configuration of materials would cause the selenium to recrystallize and to work less effectively.
In prior-art applications of imaging plates, the photoconductive layers have been responsive to a particular range of light wavelengths. In many instances, the photoconductive material used would have great responsiveness to a particular wavelength of light but have a low charge retention time. In other cases, the photoconductive material used would have a high retention time but would be limited and responsive to only particular light wavelengths. As such, the photoconductive materials used in prior-art imaging plates did not act as ideal photoreceptors. Each of the systems would have limitations when applied to particular systems of readout and particular wavelengths of light reception.
It is an object of the present invention to provide a imaging plate structure that effectively retains the charge while preventing losses due to carriers.
It is another object of the present invention to provide an imaging plate structure that allows for the readout of amplitude output.
It is another object of the present invention to provide an imaging plate that is responsive to a wider range of light wavelengths while providing for high retention times.
It is still a further object of the present invention to provide an imaging plate structure that is suitable for large-area imaging systems.
It is still another object of the present invention to provide an imaging plate structure that has an outer conductive layer with relatively low capacitance.
Another object of the present invention is to provide an imaging plate structure that maximizes the signal-to-noise ratio during readout.
It is still another object of the present invention to provide an imaging plate structure that is relatively easy to manufacture.
These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims.